Those skilled in the automatic transmission art have long directed considerable attention to improving the quality of a shift or change in gear ratio provided in an automatic transmission. A too brief duration shift rapidly accelerates or decelerates the vehicle, producing a noticeable jerk which is objectionable. If the shift time extends for too long a period, the friction elements of the transmission are subjected to undue wear, and in addition the shift has an unpleasant feel. In general, the optimum shift quality is realized with a shift time duration somewhere between the too-short and too-long limit conditions just described. A comprehensive treatment of this subject has been provided by F. J. Winchell and W. D. Route in "Ratio Changing the Passenger Car Automatic Transmission", which appears as Chapter 10 in the SAE publication "Design Practices--Passenger Car Automatic Transmissions", Copyright by the Society of Automotive Engineers, Inc., 1973. In particular, FIGS. 21 and 25 of this chapter depict speed, torque and pressure variations during a power-on upshift and during a power-on downshift.
Considerable research has been directed to the effective control of the shift itself, and specifically toward enhancing the shift quality by providing a closed loop system using a torque signal for regulating the change of gear ratio during the upshift. Details of the feedback system using the torque transducer and the appropriate logic signals are described and claimed in U.S. Pat. No. 4,031,728, entitled "Method and Apparatus for a Transmission Control System", which issued to Alan Leonard Miller and John Saxon Ivey on June 28, 1977, and which is assigned to the assignee of this invention. Subsequently this basic control system was improved to provide regulation of a friction-to-friction downshift, in addition to the upshift control set out in the patent noted above. This improved arrangement is described and claimed in U.S. Pat. No. 4,102,222, entitled "Transmission Control System", which issued to Alan Leonard Miller and Werner Paul Petzold on July 25, 1978, and which is assigned to the assignee of the present invention.
Additional work has been directed to ratio changing in an automatic transmission, starting from the systems set out in the two patents noted above. Of course, the straight-forward approach would be to aggregate the systems described in the patents, with an individual control arrangement for each upshift and each downshift. However this leads to the incorporation of a large number of circuits which are virtually identical, but can only be used in a particular upshift or downshift operation. Thus a considerable effort has been directed to effecting an optimum trade-off between the number of redundant circuits and the increase of logic complexity consequent upon reducing the number of circuits.
It is thus a primary object of the present invention to provide an efficient, low-cost control system for regulating the gear ratio changes in an automatic transmission, where multiple ratios must be provided and both the upshift and the downshift must be regulated.
Another important object of the invention is to provide a novel and unobvious method of operation in which the present gear ratio is compared with the desired gear ratio, and upon a disparity between the actual and the desired ratios, the comparison is interrupted while a ratio change is effected, and then the comparison is resumed.
A corollary object of the present invention is to provide such a system with the greatest simplicity and economy, by assigning circuits which are then used in regulating a gear ratio change, and thereafter releasing the assigned circuits and resuming the comparison function.